Measurement of low optical losses of an absorbing medium, such as a gas or a molecular beam, may be performed by measuring the ring-down time, or decay time, of a light pulse as the pulse makes multiple passes through the medium. Such measurements may be carried out in a ring-down cavity consisting of two or more mirrors, between which the light pulse is reflected, and in which the absorbing medium under test is disposed. The cavity can also be used to characterize the mirrors when no absorbing medium is present. See, for example, Romanini et al., 1993; Scherer et al., 1997; Berden et al., 2000; Lehmann, U.S. Pat. No. 5,528,040, issued Jun. 18, 1996.
Cavity ring-down spectroscopy (CRDS) is well established as a gas phase measurement method. Recently CRDS was shown to be applicable to absorption measurements on liquid samples, in which a high finesse cavity was either filled entirely with a liquid sample (Hallock et al., 2002) or the liquid was contained in a cuvette (Xu et al., 2002).
Ring-down spectroscopy using optical fibers rather than a cavity was attempted by von Lerber et al. (2002), who deposited highly reflective coatings onto both end facets of a 10 m optical fiber. Stewart et al., (2001) inserted a gas phase absorption cell into a fiber-loop, leading to very high transmission losses. These losses necessitated the use of a fiber amplifier, and the sensitivity of measurements using such an active loop depended strongly on the amplifier's temporal stability.
We have previously demonstrated a fiber-loop ring-down technique for characterizing low-loss processes in optical systems and for spectroscopy of minute liquid samples (Loock et al. U.S. Pat. No. 6,842,548, issued Jan. 11, 2005), based on measuring the ring-down time of a light pulse injected into the loop. Although extremely sensitive, limitations of that technique include slow data acquisition rate and high cost of optical components such as fast pulsed lasers.